Non-magnetic package and method of manufacture

ABSTRACT

A non-magnetic hermetic package includes walls that surround an open cavity, with a generally planar non-magnetic and metallic seal ring disposed in a continuous loop around upper edges of the walls; a sensitive component that is bonded within the cavity; and a non-magnetic lid that is sealed to the seal ring to close the cavity by a metallic seal.

BACKGROUND

1. Technical Field

Embodiments of the invention relate generally to packaging of devicesthat are sensitive to temperature, humidity, and/or atmosphericconstituents. Particular embodiments relate to packages formicro-electrical-mechanical systems (“MEMS”) that are suitable for usein magnetic-resonance imaging (“MRI”) equipment and in otherapplications that require non-magnetic components.

2. Discussion of Art

Micro-electro-mechanical systems (“MEMS”) are devices that have theirlargest dimension within a range from about 20 micrometers to about 1millimeter (0.02-1.0 mm). These very small electrical machines areuseful in many applications, for example: ejecting ink from thecartridges of inkjet printers to put letters onto a page; measuringaccelerations of a vehicle or of a handheld device such as a cellularphone or a game controller; transducing air pressure waves or surfacevibrations to record sound; switching optical signals among fiberarrays; etc.

Generally, MEMS are useful for reliably providing highly responsive(small time constant) electromechanical functionality, such as motionsensing, within a small footprint or volume envelope. Accordingly, ithas been desired for several years to make use of MEMS for sensing andcontrol within MRI systems. However, it is necessary in MRI systems toprovide components that are “non-magnetic,” i.e., neither ferromagneticnor paramagnetic.

To date, MEMS packaging has relied upon materials that are eitherferromagnetic and/or paramagnetic. This has been the case in partbecause packages fabricated from non-magnetic materials, e.g., ceramicsor plastics, have been understood to require closure methods that riskedthermal damage, e.g., brazing, and/or chemical damage, e.g., volatile ormoisture-permeable adhesives, to the MEMS enclosed within the packages.MEMs are sensitive to their environment, particulate or chemically, andalso are sensitive to the packaging processing conditions, thus the needfor a process to control the conditions and package environment. Indeed,MEMS typify a category of “sensitive” components that require specialcare in packaging. Other constituents of this category may includepiezoelectric, paramagnetic, and shape memory alloy devices.

In view of the above, it is desirable to provide a sensitive componentwithin a non-magnetic hermetic package. A key difficulty in providingsuch a package has been to devise a method of bonding a sensitivecomponent into an open non-magnetic package cavity, then sealing thepackage cavity, without inducing thermal and/or chemical damage to thesensitive component.

BRIEF DESCRIPTION

Embodiments of the invention provide a non-magnetic hermetic MEMSpackage, which includes a non-magnetic ceramic package body having wallsthat surround a cavity, with a generally planar non-magnetic andmetallic seal ring disposed around upper edges of the walls; anundamaged MEMS device that is bonded to the package body within thecavity; and a non-magnetic lid that is sealed to the seal ring of thepackage body to close the cavity by an hermetic metallic seal.

Other embodiments of the invention provide a non-magnetic ceramicpackage body, which includes walls that surround a cavity; a generallyplanar non-magnetic and metallic seal ring disposed in a continuous looparound upper edges of the walls; and a MEMS device bonded to the packagebody within the cavity.

Other aspects of the invention provide for making a non-magnetichermetic MEMS package according to a method that includes forming anon-magnetic ceramic package body having walls that surround an opencavity; installing a non-magnetic and generally planar metallic sealring in a continuous loop around upper edges of the package body walls;bonding a MEMS device to the package body within the cavity; and sealinga non-magnetic lid to the seal ring.

Yet other aspects of the invention provide a method for fluxless solderbonding a MEMS device to a package body, which includes providing anindium preform between facing surfaces of the MEMS device and of thepackage body; without introducing flux, heating and holding the indiumpreform within a reflow temperature range; and scrubbing the MEMS deviceagainst the indium preform to reflow and adhere the indium preform tothe MEMS device and to the package body.

DRAWINGS

The present invention will be better understood from reading thefollowing description of non-limiting embodiments, with reference to theattached drawings, wherein below:

FIG. 1 is a perspective partially sectioned view of a non-magneticpackage including an exemplary non-magnetic lid that is sealed to anon-magnetic package body, according to embodiments of the invention.

FIG. 2 is a perspective partially sectioned view of the non-magnetic lidof FIG. 1.

FIGS. 3A-3C are perspective views of a glove box, tacker, oven, and seamsealer used to fabricate an embodiment of the invention.

FIG. 4 is a partial sectioned detail view of a seam-seal between thenon-magnetic lid and the non-magnetic package body of FIG. 1.

DETAILED DESCRIPTION

Reference will be made below in detail to exemplary embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference characters usedthroughout the drawings refer to the same or like parts, withoutduplicative description. Although exemplary embodiments of the presentinvention are described with respect to their use in MRI systems,embodiments of the invention are applicable for use in any setting thatcan benefit from non-magnetic switches or sensors.

As used herein, the terms “substantially,” “generally,” and “about”indicate conditions within reasonably achievable manufacturing andassembly tolerances, relative to ideal desired conditions suitable forachieving the functional purpose of a component or assembly.

In an exemplary embodiment, as shown in FIG. 1, a non-magnetic MEMSpackage body 200 includes a wall 201 that is topped by the generallyplanar metallic seal ring 250. The wall 201 encloses a cavity 204.Within the cavity 204, a MEMS (micro-electro-mechanical) device 210 ismounted within the package body 200 to a floor 206, which can havemultiple levels or steps, by a low temperature, fluxless indiumsoldering process. An exemplary “low temperature” soldering processincludes placing an indium preform 212 into the package body 200,placing the package body onto a bonding stage, placing the MEMS device210 onto the indium preform, heating the MEMS device and package body toa reflow temperature of about 150° C. but no more than about 200° C.,holding at the reflow temperature for at least about 15 seconds, andcooling down from the reflow temperature (optionally, with use of acooling blow). Typically, either or both of the undersurface(“backside”) of the MEMS device 210, as well as the mating region of thepackage body floor 206, will have been pre-coated with a non-magneticsolderable coating (not shown) such as TiW/Au, W/Cu/Au, or W/Cu/Pd/Au.

MEM structures can be fabricated from temperature sensitive materials oralloys such as AuNi, NiW, or the like, which tend to degrade or deformabove about 200° C. Thus, soldering at low temperature, e.g., with abonding stage head temperature not exceeding about 210° C., avertsdamage to the MEMS device 210. Moreover, soldering without use of fluxeliminates a potential source of volatile chemicals from the fluxoff-gas sing, thus, enables a chemically inert dry air environment to bemaintained within the MEMS package 400 after seam sealing. However,without use of flux, which typically lowers breaks down oxides andsurface contamination to improve wetting/surface tension and surfaceadhesion of solder, it can be difficult to obtain a good solder bondwith the oxide impurities still in place.

Accordingly, during the hold at reflow temperature, a pick and placetool is used to scrub or oscillate the MEMS device 210 across the indiumpreform, thereby breaking up the oxides and enhancing reflow of thepreform in order to achieve a good mechanical bond between the MEMSdevice and the package body 200, without use of flux or inertatmosphere, ie forming gas. The scrubbing motion of the MEMS device 210may be purely lateral reciprocation, or can be a swirling orside-stepped motion or a random motion within an areal envelope orstrictly in Z motion, i.e. as a tapping motion or even as a singleup-and-down motion that may break surface tension and initial adhesionof a solder oxide layer to the die. The scrubbing motion of the MEMSdevice 210 may be accomplished during a dwell stage of the reflowtemperature profile. In an embodiment with the MEMS device being about 3mm square, contact load of the pick and place tool against the MEMSdevice 210 may be at least about 0.2 newton in order to achieve reflow,but may be not more than about 6 newtons in order to avoid exceedingdesign loads on the MEMS device or substrate. In certain embodiments,contact load may be about 5 newton. Appropriate contact load will varyaccording to the size and durability of the MEMS device 210. The picktool is in contact with the MEMS device 210 throughout the scrubbingmotion, and the pick tool head is heated to aid in heat transfer to thesolder through the MEMS device. For example, the pick tool head may beheated to at least about 150° C., or as much as about 180° C. forthorough heating of the solder adjacent the MEMS device, but may beheated to no more than about 210° C. in order to avert damage to theMEMS device 210.

In an embodiment, the seal ring 250 is formed in three layers: atransition layer 252 of tungsten, a seed layer 254 of copper, and a seallayer 256 of gold. Each of the seal ring layer materials is selected foroptimal bonding to adjacent layers, e.g., the tungsten transition layer252 is optimally suited for bonding to the ceramic wall 201, while thecopper seed layer 254 is optimally compatible both with the tungsten andwith the gold seal layer. Due to temperature limitations of the MEMSdevice 210, the seal ring 250 is formed at the upper edge of the ceramicwall 201, before the MEMS device is mounted into the cavity 204.Optionally, a layer of palladium can be provided between the copper andthe gold.

Once the MEMS device 210 has been mechanically solder bonded onto thepackage body 200, at the floor 206 of the cavity 204, then the MEMSdevice 210 can be electrically wirebonded to contacts 220, which extendfrom inside the cavity 204 through the package body 200 to external pins222. In certain embodiments, as-installed capacitance between each pairof external pins 222, through the installed MEMS device 210, is lessthan about 2 pF. As an alternative to the solder bonding andwirebonding, steps as described above, external gold points of the MEMSdevice 210 may instead be ultrasonically welded to the gold electricalcontacts 220 that are formed on the package body.

Following wire bonding, the MEMS package body 200, the MEMS device 210,and the lid 100 (prior to attachment of solder preforms) may be vacuumbaked for removal of moisture and other volatiles. For example, a 100°C./30 minute dwell may be implemented in a vacuum oven. Immediatelyafter vacuum baking, the package body 200 and the MEMS device 210 may beashed using a commercially available oxygen ashing process.

Within about no more than one hour after oxygen ashing of the packagebody 200, and typically within no more than about 30 minutes afterashing, the lid 100 is joined to the package body by seam sealing (withan optional preliminary step of tacking). The seam sealing and tackingare accomplished within a glove box environment filled with clean “dryair,” i.e., standard air with particulates filtered out and with watervapor removed to dew point of less than −40° C. or less than 10,000 ppm.For example, clean dry air may be filtered to meet at least ISO Class 8;in some embodiments, at least ISO Class 6; in certain embodiments, atleast ISO Class 4. It should be noted that seam sealing under air, whichincludes a substantial fraction of oxygen (about 21%), can helpfultoward achieving certain performance targets for as-built devices.

For example, in certain embodiments, on-resistance of the MEMS device210 drifts no more than 1 ohm after an initial burn-in. Low drift can beachieved because the hermetic seal of clean dry standard air, within thepackage 400, results in an equilibriated oxidation of the MEMS switchdevice contacts and other surfaces. Whereas equilibriated oxidationcannot occur in an unsealed package, it also cannot occur withinoxygen-free (e.g., inert gas or vacuum) sealed packages. Theequilibriated oxide layer acts to stabilize mechanical and electricalperformance of MEMS switch contacts, thereby maintaining on-resistancedrift to less than 1 ohm.

Still referring to FIG. 1, a non-magnetic lid 100 (of which an exemplaryembodiment is shown, in greater detail, in FIG. 2) can be seam-sealedonto the non-magnetic MEMS package body 200 to provide an hermeticnon-magnetic MEMS package 400. The non-magnetic lid 100 includes, at aminimum, a molybdenum substrate 110 and a gold/tin solder preform 150that extends around an outer periphery 102 of a sealing surface 104 ofthe lid. FIG. 4 shows a partial sectioned schematic of the hermeticseam-seal 450 that is formed between the preform 150 of the lid 100 andthe seal ring 250 of the package body 200, as discussed above. As usedherein, “hermetic” means obtaining a helium leak rate of 3×10̂−8 mBarrL/s or less using a helium fine leak tester (e.g., an Alcatel™ ASM180model) after two hours hold under at least 60 psi helium; and showing nobubbles in 125C solution of FC-40; or equivalent test results.

Referring now to FIGS. 3A-3C, the non-magnetic lid 100 is sealed ontothe non-magnetic MEMS package body 200 by using a seam sealer 300 and atack welder 320, which are housed within a dry air glove box 310. Theseam sealer 300 applies electrical current from its electrodes 302progressively to the lid 100 and to the patterned metallic seal ring 250of the package body 200, thereby locally reflowing the Au/Sn preform andsealing together the lid 100 with the metallic seal ring 250. Notably,adhesion of the palladium layer 140, and/or the gold coating 160, aroundthe edge 102 of the lid 100 enables a fillet 450 to extend up over theedge of the lid, thereby enhancing sealing of the seam.

Referring particularly to FIG. 2, an exemplary embodiment of the lid 100includes a molybdenum substrate 110. For example, the substrate 110 maybe stamped or cut to a flat shape, or it may be stamped or drawn to aconcave or stepped shape. The substrate is at least about 5 mil (0.005inch) thick, usually about 10 mil thick, but can be thicker. Generally,a minimum thickness for the substrate 110 is set by a desire to maintaindry air (e.g., less than about 10,000 ppm water vapor, or less than −40°C. dew point) at an atmospheric pressure within the package 400, evenduring depressurized air cargo transport, without permanent deformationof the substrate; a maximum thickness is limited by practicability ofmanufacture, and by a generic desire to reduce total assembly weight.The molybdenum substrate also desirably provides for thermal andelectrical bulk resistivities that will facilitate seam-sealing bylocalized electrical heating, as described above.

The exemplary embodiment of the lid 100 also includes, over themolybdenum substrate 110, an adhesion layer 120 that is at least about500 angstrom thick and can be up to about 2000 angstrom thick. Theadhesion layer can be sputtered onto the substrate 110 from titanium orfrom similar molybdenum-compatible metals such as tantalum or chromium.In certain embodiments, equivalent methods of physical vapor deposition(PVD) can be used in place of sputtering. PVD processes include cathodicarc, electron beam, resistive evaporative, pulsed laser, and magneticsputtering deposition techniques. In certain embodiments, magneticsputtering is the coating method, primarily due to chemical difficultiesin electroplating or chemical vapor deposition (CVD). A sputteredadhesion layer has been found more uniform and provide better adhesionthan a plated or CVD adhesion layer, and sputter systems provide backsputter capabilities to improve adhesion further. Other PVD techniquesare believed equally suitable by comparison to electrochemical or CVDtechniques. The adhesion layer 120 is sputtered onto opposite broadsurfaces of the substrate, and is sputtered at least thick enough thatthe edges of the sputtered regions join to cover the edges of thesubstrate; thus, minimum acceptably thickness of the adhesion layer willvary according to thickness of the substrate. The adhesion layer 120generally is not much thicker than needed to cover the edges of thesubstrate from the two broad surfaces.

Over the adhesion layer 120, the lid has a copper seed layer 130 that issputtered to at least about 1000 angstrom thick. Copper has been foundsuitable for the seed layer 130 in terms of its compatibility with thetitanium/tantalum/chromium adhesion layer, and in terms of itsacceptance for palladium electroplating. The seed layer 130 is notdirectly sputtered onto the molybdenum because the adhesion layer 120has been found to greatly enhance attachment of the seed layer 130 tothe substrate 110. In some embodiments the seed layer 130 may be about2000 angstrom thick, and in certain embodiments the seed layer may be asmuch as about 6000 angstrom thick. A thicker seed layer helps to bufferback-etch from a subsequent step of electroplating, and helps to coveredges of the substrate. On the other hand, increased thickness adds costand weight, and an excessive thickness of copper, which is highlythermally conductive, may present problems with a later step ofseam-sealing the lid to close the MEMS package.

Outside the seed layer 130, the lid 100 includes a palladium solder baselayer 140 that is about 1-2 μm thick, where again the thickness ischosen to ensure that edges of the substrate 110 are covered. Thepalladium solder base layer 140 typically is electroplated onto thecopper seed layer 130. Accordingly, the palladium solder base layer 140will cover all surfaces of the lid 100 so as to mitigate tarnishing ofthe copper seed layer. In other embodiments, the solder base layer 140may be sputtered onto just one surface (a sealing surface 104) of thelid 100.

On the sealing surface 104 of the lid 100, near its edges, the gold/tinsolder preforms 150 then are attached to the solder base layer 140,e.g., by tack welding. Overall, the materials and various layerthicknesses of the lid 100 are chosen so that the lid has bulkresistivity sufficiently high to focus current for spot seam sealing ofthe solder preforms 150, as opposed to welding the lid itself. Thus, theinventive lid 100 allows for a low power seam sealing process that doesnot impose bulk heating of the lid or of a structure to which the lid isattached. Therefore, it is possible to use the lid for hermetic sealingof a MEMS package, without risking thermal damage to a MEMS devicewithin the package. The lid 100 also may include a gold flash layer 160between the palladium solder base layer 140 and the solder preforms 150.

Referring again to FIG. 3, the steps of tacking and seam sealing areaccomplished within a glove box 310, which maintains clean air under dryconditions (less than about −40° C. dew point, 0% relative humidity,less than about 10,000 ppm water vapor). During tacking, the lid 100 isplaced onto the package body 200 and the two components are placedtogether into a tack-welder 320 within the glove box. The tack welder320 is used to lightly press down the lid 100 and to tack the lid withthe gold/tin solder preform 150 onto the metallic seal ring 250 at oneor two locations. Before tacking, the lid 100 and the package body 200may be O2 ashed/cleaned and then purged with CDA (clean dry air asdiscussed below) and heated in a vacuum oven 330, which is connectedwith and opens into the glove box 310. For seam sealing, the packagebody 200 is transferred within the glove box onto a rotatable stage 304of the seam sealer 300.

The seam sealer 300 then is operated to rotate the stage 304 with thelid and body, while retracting and extending the electrodes 302, so asto move the electrode contact along the edge 102 of the lid 100 that thepreform 150 and the seal ring 250 will be progressively locally heatedand welded to form an hermetic weld or solder fillet between the solderbase layer and an opposed seal ring. In this regard it is helpful tohave the edges of the substrate 110 fully covered by the solder baselayer 140 and subordinate layers 120, 130, so as to form a uniformfillet 450, as shown in FIG. 4 (need to update figure to show filletgoes up side edge of lid). Additionally it is helpful to have the lidand package body tacked together at only one location, where theelectrodes 302 will start the seam-sealing process.

According to an aspect of the invention seam sealing, which is alocalized resistance welding/heating process, is usable as opposed tobrazing (which is well known for sealing solder preforms). Althoughbrazing and furnace solder reflow have been conventional for sealingpackages such as the inventive package 400, it turns out that thegeneralized high temperatures obtained during brazing can damage anddeform the MEMS device that is meant to be enclosed within the hermeticMEMS package. Also in furnaces it is difficult to establish a CDAenvironment. Furnaces typically have forming gas or inert atmosphere forconvective heating, the glove box and seam sealing eliminate need forconvective heating and thereby enable use of clean dry air. However, inorder to implement seam sealing, it is necessary that the lid 100 mustenable localized heating resistively. Many non-magnetic metals make itvery difficult to provide heat only in a local area, because such metalstend to be highly thermally and/or electrically conductive. Therefore,the materials and layer thicknesses of the lid 100 are carefullyselected in order to optimize seam sealing, in a manner not previouslyconsidered.

Processes as described above result in an hermetic and non-magnetic MEMSpackage 400, which is usable in various applications including withinMRI enclosures. Because dry air at atmospheric pressure is capturedinside the package 400, it also may be usable in other environmentswhere MEMs have not previously been considered.

Embodiments of the invention provide a non-magnetic hermetic package,which includes walls that define an interior cavity, with a generallyplanar non-magnetic and metallic seal ring disposed around upper edgesof the walls; at least one electrical contact in the interior cavityextending from the interior through the walls to an external pin; anundamaged sensitive component that is bonded to the package body withinthe cavity; and a non-magnetic lid that is sealed to the seal ring ofthe package body to close the cavity by an hermetic metallic seal. Theseal ring may include a tungsten layer that is bonded to the packagebody walls, a copper layer over the tungsten layer, and a gold layerover the copper layer, with the gold layer bonded to the metallic seal.Alternatively, the seal ring could include W/Cu/Pd/Au layers. Themetallic seal may be formed by gold/tin solder. For example, themetallic seal may be formed by seam-sealing the gold/tin solder to agold layer of the metallic seal ring. Thus, the non-magnetic lid mayinclude, at a sealing surface of the lid, gold/tin solder preforms. Thepackage also may include electrical contacts inside the cavity that areelectrically connected with the sensitive component, and further mayinclude external pins formed on the package walls and electricallyconnected with each other via the sensitive component and the electricalcontacts inside the cavity, the capacitances among the external pins viathe sensitive component being less than about 2 pF. The sensitivecomponent may be a MEMS device, in particular a switch that has stableon-resistance that drifts less than 1 ohm after burn-in, and/or ablocking voltage of at least about 250 V. The non-magnetic lid may beseam sealed to the seal ring in clean dry air with approximatelyatmospheric oxygen concentration.

Other embodiments of the invention provide a non-magnetic ceramicpackage, which includes walls that surround a cavity; a generally planarnon-magnetic and metallic seal ring around upper edges of the walls; andat least one electrical contact in the interior cavity extending fromthe interior through the walls to an external pin. The seal ringincludes a tungsten layer that was bonded to the package body walls, acopper layer over the tungsten layer, and a gold layer over the copperlayer. The seal ring also may include a palladium layer between the goldand copper layers. The sensitive component may have been bonded to thepackage body using a fluxless low temperature solder process. Indium mayhave been used in the solder process.

Other aspects of the invention provide for making a non-magnetichermetic package according to a method that includes forming anon-magnetic ceramic package body having walls that surround an opencavity and having a non-magnetic and generally planar metallic seal ringin a continuous loop around upper edges of the package body walls;bonding a sensitive component to the package body within the cavity; andsealing a non-magnetic lid to the seal ring. Installing the seal ringmay include forming the non-magnetic and generally planar metallic sealring by first forming a tungsten layer to be bonded to the package bodywalls, then depositing a copper layer onto the tungsten layer, and thendepositing a gold layer over the copper layer. In some aspects,installing the seal ring may include depositing a tungsten layer in acontinuous loop around the upper edges of the package body walls, thendepositing a copper layer onto the tungsten layer, and then depositing agold layer over the copper layer. Bonding the sensitive component mayinclude a fluxless low temperature solder process, in which indium maybe used as the low temperature solder. The low temperature solderprocess may include a scrubbing motion of the sensitive component withreference to the package body. Certain aspects of the method may includepre-coating facing surfaces of the sensitive component and of thepackage body cavity with a non-magnetic solderable coating, whichconsists essentially of TiW/Au or of W/Cu/Au. Sealing the non-magneticlid may be carried out as a seam-sealing process. Optionally, thepackage body and sensitive component may be ashed and/or vacuum bakedbefore sealing the non-magnetic lid to the non-magnetic metallic sealring. Optionally, seam-sealing the non-magnetic lid may be accomplishedunder dry air at about 1 atm. Additionally, the package body, sensitivecomponent, and lid may be heated in an oven under dry air at about 1 atmbefore sealing the non-magnetic lid to the non-magnetic metallic sealring.

Other aspects of the invention provide a method for fluxless solderbonding a MEMS device to a package body, which includes providing anindium preform between facing surfaces of the MEMS device and of thepackage body; without introducing flux or forming gas, heating andholding the indium preform within a reflow temperature range; andscrubbing the MEMS device against the indium preform to reflow andadhere the indium preform to the MEMS device and to the package body.The method also may include pre-coating at least one of the facingsurfaces with a non-magnetic solderable coating. For example, thenon-magnetic solderable coating may include either TiW/Au or W/Cu/Au.The reflow temperature range may be between about 150° C. and about 210°C. For example, the indium preform may be held at about 200° C. but notmore than about 210° C. The indium preform may be heated via the packagebody and via the MEMS device.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (and/or aspects thereof) may be used in combination witheach other. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the inventionwithout departing from its scope. While the dimensions and types ofmaterials described herein are intended to define the parameters of theinvention, they are by no means limiting and are exemplary embodiments.Many other embodiments will be apparent to those of skill in the artupon reviewing the above description. The scope of the invention should,therefore, be determined with reference to the appended claims, alongwith the full scope of equivalents to which such claims are entitled. Inthe appended claims, the terms “including” and “in which” are used asthe plain-English equivalents of the respective terms “comprising” and“wherein.” Moreover, in the following claims, terms such as “first,”“second,” “third,” “upper,” “lower,” “bottom,” “top,” etc. are usedmerely as labels, and are not intended to impose numerical or positionalrequirements on their objects. Further, the limitations of the followingclaims are not written in means-plus-function format and are notintended to be interpreted based on 35 U.S.C. §112, sixth paragraph,unless and until such claim limitations expressly use the phrase “meansfor” followed by a statement of function void of further structure.

This written description uses examples to disclose several embodimentsof the invention, including the best mode, and also to enable one ofordinary skill in the art to practice embodiments of the invention,including making and using any devices or systems and performing anyincorporated methods. The patentable scope of the invention is definedby the claims, and may include other examples that occur to one ofordinary skill in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral language of the claims.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralof the elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising,”“including,” or “having” an element or a plurality of elements having aparticular property may include additional such elements not having thatproperty.

Since certain changes may be made in the above-described article andmethod of making, without departing from the spirit and scope of theinvention herein involved, it is intended that all of the subject matterof the above description or shown in the accompanying drawings shall beinterpreted merely as examples illustrating the inventive concept hereinand shall not be construed as limiting the invention.

What is claimed is:
 1. A non-magnetic hermetic package comprising: wallsthat surround an open cavity, with a generally planar non-magnetic andmetallic seal ring disposed in a continuous loop around upper edges ofthe walls; a sensitive component that is bonded within the cavity; and anon-magnetic lid that is sealed to the seal ring to close the cavity bya metallic seal.
 2. The package as claimed in claim 1, wherein the sealring includes a tungsten layer that is bonded to the package body walls,a copper layer over the tungsten layer, and a gold layer over the copperlayer, with the gold layer bonded to the metallic seal.
 3. The packageas claimed in claim 1, wherein the seal ring includes a tungsten layerthat is bonded to the walls, a copper layer over the tungsten layer, apalladium layer over the copper layer, and a gold layer bonded to themetallic seal.
 4. The package as claimed in claim 1, wherein themetallic seal is formed by gold/tin solder.
 5. The package as claimed inclaim 1, further comprising electrical contacts inside the cavity thatare electrically connected with the sensitive component.
 6. The packageas claimed in claim 5, further comprising external pins formed on thepackage walls and electrically connected with each other via thesensitive component and the electrical contacts inside the cavity, thecapacitances among the external pins via the sensitive component beingless than about 2 pF.
 7. The package as claimed in claim 1, wherein thesensitive component is a MEMS device.
 8. The package as claimed in claim7, wherein the MEMS device is a switch that has stable on-resistancethat drifts less than 1 ohm after burn-in.
 9. The package as claimed inclaim 7, wherein the MEMS device is a switch that has blocking voltageof at least about 250 V.
 10. The package as claimed in claim 1, whereinthe non-magnetic lid is seam sealed to the seal ring in clean dry airwith approximately atmospheric oxygen concentration.
 11. The package asclaimed in claim 1, wherein the package contains gaseous oxygen.
 12. Thepackage as claimed in claim 11, wherein the package contains clean dryair with dewpoint less than about −40° C.
 13. The package as claimed inclaim 1, wherein the lid is non-magnetic.
 14. The package as claimed inclaim 13, wherein the lid is metallic.
 15. The package as claimed inclaim 14, wherein the lid includes molybdenum.
 16. A non-magneticpackage comprising: walls that define an interior cavity; a generallyplanar non-magnetic and metallic seal ring around upper edges of thewalls; and at least one electrical contact in the interior cavityextending from the interior through the walls to an external pin,wherein the seal ring includes a tungsten layer, a copper layer, apalladium layer, and a gold layer.
 17. The package as claimed in claim16, further comprising a sensitive component that is bonded into thecavity using a fluxless low temperature process.
 18. The package asclaimed in claim 17, wherein indium solder was used in the bondingprocess.
 19. The package as claimed in claim 18, wherein the bondingprocess included a scrubbing motion of the sensitive component withinthe package body.
 20. The package as claimed in claim 17, wherein thebonding process included ultrasonic welding of the sensitive componentto the at least one electrical contact.
 21. A method for making anon-magnetic hermetic package, said method comprising: obtaining anon-magnetic ceramic package body having walls that surround an opencavity and having a non-magnetic and generally planar metallic seal ringin a continuous loop around upper edges of the package body walls;bonding a sensitive component within the cavity; and sealing anon-magnetic lid to the seal ring.
 22. The method as claimed in claim21, wherein bonding the sensitive component comprises a fluxless lowtemperature solder process.
 23. The method as claimed in claim 22,wherein indium is used as the low temperature solder.
 24. The method asclaimed in claim 21, wherein the bonding process includes a scrubbingmotion of the sensitive component with reference to the package body.25. The method as claimed in claim 21, wherein sealing the non-magneticlid is accomplished under oxygen.
 26. The method as claimed in claim 21,wherein sealing the non-magnetic lid is a seam-sealing process.
 27. Themethod as claimed in claim 21, further comprising pre-coating facingsurfaces of the sensitive component and of the cavity with anon-magnetic solderable coating, which consists essentially of TiW/Au orof W/Cu/Au.
 28. The method as claimed in claim 21, further comprisingashing and vacuum baking the package body and the sensitive componentbefore sealing the non-magnetic lid to the non-magnetic metallic sealring.
 29. A method for fluxless solder bonding a MEMS device to apackage body, comprising: providing an indium preform between facingsurfaces of the MEMS device and of the package body; without introducingflux or forming gas, heating and holding the indium preform within areflow temperature range; and scrubbing the MEMS device against theindium preform to reflow and adhere the indium preform to the MEMSdevice and to the package body.